As semiconductor devices using nitride semiconductors, many reports on field effect transistors, especially, high-electron-mobility transistors (HEMT) have been made. As a HEMT using nitride semiconductors, a HEMT that uses a GaN layer for a channel layer and an AlGaN layer for a barrier layer has been known. In such a GaN-based HEMT, strain caused by a difference between the lattice constants of AlGaN and GaN occurs in the AlGaN layer; this strain brings a piezoelectric polarization; and a highly-concentrated two-dimensional electron gas is generated near the upper surface of the GaN layer under the AlGaN layer. For this reason, a high output can be obtained.
In order to improve the mobility, quantum-confinement-structure transistors that use a great band offset between AlN and GaN have also been proposed.
However, conventionally, it has been difficult to practically manufacture a quantum-confinement-structure transistor having a high quantum confinement effect.